Steel parts of cars are often exposed to environments where atomic hydrogen can be formed and absorbed. The absorbed hydrogen may be in addition to what has already been absorbed during component manufacture. The detrimental effects that hydrogen can cause in steel are: reduce the failure stress of steel, limit ductility and toughness, or even accelerate crack growth within the steel. The failure of steel due to hydrogen attack may occur instantaneously upon loading or after a delayed period of time. This behavior makes it exceptionally difficult to predict failures due to hydrogen embrittlement and can be costly from the standpoint of liability and repairs. In general, susceptibility to hydrogen degradation increases with increasing steel strength, and is more pronounced when the strength of the steel is greater than 1000 MPa.
Thus, several families of steels like the ones mentioned below offering various strength levels have been proposed.
Among those concepts, steels with micro-alloying elements whose hardening is obtained simultaneously by precipitation and by refinement of the ferritic grain size have been developed. The development of such High Strength Low Alloyed (HSLA) steels has been followed by those of higher strength called Advanced High Strength Steels which keep good levels of strength together with good cold formability such as dual phase steels, bainitic steels, TRIP steels but the tensile strength levels that can be reached by such concepts is generally below 1300 MPa.
So as to answer to the demand of steels with even higher strength and at the same time a good formability, a lot of developments took place with, as a challenge, obtaining a steel grade that can withstand hydrogen embrittlement. It leads to martensitic steels with more than 1500 MPa of resistance but delayed fracture issues due to the presence of hydrogen in the steel occurred. In addition, martensitic steels present low formability levels.
The development of martensitic steels is illustrated, for instance, by the international application WO2013082188, such application deals with martensitic steel compositions and methods of production thereof. More specifically, the martensitic steels disclosed in this application have tensile strengths ranging from 1700 to 2200 MPa. Most specifically, the invention relates to thin gage (thickness of 1 mm) and methods of production thereof. However such application is silent when it comes to delayed fracture resistance, it does not teach how to obtain delayed fracture resistant steels.
It is also known the following article “ISIJ 1994 (vol 7)—Effect of Ni, Cu and Si on delayed fracture properties of High Strength Steels with tensile strength of 1450 by Shiraga” which teaches positive effect of Ni content on delayed fracture resistance due to hydrogen. However, such document would not result in enough delayed fracture resistance.